The present invention relates generally to primers, and, in particular, a primer which may be reversibly switched between adhesion and deadhesion, and to a method of preventing metal corrosion using the primer.
In the case of conventional primers (adhesion promoters, for example, isocyanate compounds) an irreversible bonding to surfaces via covalent bonds takes place. During bond-breaking reactions of these primers, the structural elements of the primers are irreversibly destroyed. In addition, the amount of energy needed for bond-breaking/destroying the known surface primers is relatively large. Therefore, it is not possible to switch back and forth between adhesion and deadhesion.
U.S. Pat. No. 4,022,649 describes a metal laminate having high thermal stability which is produced by forming a cured film layer of a thermally stable heterocyclic polymer such as polyamide imide having only a small content of volatile components on at least one surface of one or both of metal plates or foils, for example, of aluminum, separately forming an adhesive layer of a thermally stable heterocyclic polymer having a thickness of one-third or less of the above mentioned film layer and containing a volatile matter in an amount of not more than 20% by weight, and then heat bonding both of the metal plates or foils through the above mentioned adhesive layer with the film layer being oriented toward the adhesive layer. The described polymer layer decomposes at temperatures above 300xc2x0 C. and cannot be reversibly removed from the metal surface for the above-mentioned reasons.
Therefore, an object of the present invention is to provide a primer which can be reversibly switched back and forth between adhesion and deadhesion as well as methods for reversibly switching a primer on a metal surface between adhesion and deadhesion. The present invention provides a ligand layer on a metal surface, the ligand layer being capable of being reversibly switched between adhesion and deadhesion, and including substituted pyridine dimers and/or pyridine trimers which are selected from the group consisting of: 
R1 through R6 being able to be equal or different and being selected from the group consisting of xe2x80x94Rxe2x80x94CH(O)CH2, OH, xe2x80x94Oxe2x80x94R, NR2, NHR, NH2, CO2H, CO2R, R and H with R being selected from the group consisting of alkyl and aryl residues with the proviso that not all residues R1 through R6 are hydrogen atoms at the same time. R7 is selected from the group consisting of xe2x80x94Rxe2x80x94CH(O)CH2, OH, xe2x80x94Oxe2x80x94R, NR2, NHR, NH2, CO2H, CO2R, CONHR, R and H, with R being selected from the group consisting of alkyl and aryl residues. Y is selected from the group consisting of silicon, carbon, nitrogen, phosphorus and oxygen atoms, with X being an oxygen, sulfur or phosphorus atom, and n having the value 0 or 1.
In an embodiment of the ligand layer on a metal surface according to the present invention, R1 through R6 are selected from the group consisting of xe2x80x94Rxe2x80x94CH(O)CH2, xe2x80x94Oxe2x80x94R, CO2R, CONHR and R with R being selected from the group consisting of alkyl and aryl residues. R7 is selected from the group consisting of xe2x80x94Rxe2x80x94CH(O)CH2, xe2x80x94Oxe2x80x94R, CO2H, CO2R, NR2 and R, with R being selected from the group consisting of alkyl and aryl residues. Y is selected from the group consisting of silicon, carbon, nitrogen, phosphorus and oxygen atoms. X is an oxygen atom, and n has the value 1.
The alkyl residues may contain 1 to 30 carbon atoms.
The present invention also provides a method for reversibly switching a ligand layer on a metal surface according to the present invention between adhesion and deadhesion, including the steps of
(a) applying a primer solution;
(b) drying the primer film;
(c) applying an adhesive or lacquer;
(d) curing of the adhesive or lacquer;
(e) deadhesion of the primer/adhesive composite or primer/lacquer composite by applying a direct voltage to the composite, by changing the pH value of the composite or by changing the temperature of the composite; and
(f) possibly, repeated adhesion of the primer/adhesive composite or primer/lacquer composite by applying a direct voltage opposite in polarity to that of step (e), or by changing the pH value or the temperature of the composite.
In an embodiment of the method according to the present invention, the deadhesion or adhesion in steps (e) and, possibly, (f) are caused by applying a direct voltage in the range from 1.0 to 1,000 V, preferably 5 to 500 V, in particular 10 to 100 V.
In another embodiment of the method according to the present invention, the deadhesion or adhesion in steps (e) and, possibly, (f) are caused by changing the pH value to a range from 1 to 13, preferably a range from 5 to 10.
In yet another embodiment of the method according to the present invention, the deadhesion or adhesion in steps (e) and, possibly, (f) are caused by raising the temperature to 40 to 250xc2x0 C., preferably to 60 to 150xc2x0 C., in particular to 70 to 100xc2x0 C.
Moreover, the present invention provides a method for inhibiting metal corrosion, including the application of a ligand layer according to the present invention to a metal surface.